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Abstract:

A fuel cell system may be capable of reducing an adverse influence which
acts on a fuel cell at the time of restarting the fuel cell after
emergency shutdown of operation of the fuel cell. A fuel cell system
includes a fuel cell, a fuel gas supply unit, an oxygen-containing gas
supply unit, a storage unit that stores whether shutdown of operation of
the fuel cell is normal shutdown or emergency shutdown, and a control
unit that controls at least the fuel gas supply unit and the
oxygen-containing gas supply unit. The control unit, in emergency
shutdown, controls the fuel gas supply unit at a time of restarting the
fuel cell after the shutdown of the fuel cell so as to reduce an amount
of fuel gas supplied to the fuel cell to be less than that at a time of
restarting the fuel cell after normal shutdown.

Claims:

1-9. (canceled)

10. A method of operating a fuel cell system, comprising: controlling an
amount of fuel gas supplied to a fuel cell that generates electric power
using fuel gas and oxygen-containing gas and combusts fuel gas remaining
unused for generation of electric power, at a time of restarting the fuel
cell after emergency shutdown of operation of the fuel cell, to be less
than an amount of fuel gas supplied to the fuel cell at a time of
restarting the fuel cell after normal shutdown of operation of the fuel
cell.

11. The method of operating a fuel cell system according to claim 10,
wherein the fuel cell system includes: the fuel cell that generates
electric power using fuel gas and oxygen-containing gas, and combusts
fuel gas remaining unused for generation of electric power, on a first
end portion side thereof; a fuel gas supply unit that supplies the fuel
gas to the fuel cell; an oxygen-containing gas supply unit that supplies
the oxygen-containing gas to the fuel cell; a storage unit that, if any
shutdown of operation of the fuel cell should occur, stores whether the
shutdown of operation of the fuel cell is normal shutdown or emergency
shutdown; and a control unit that controls at least the fuel gas supply
unit and the oxygen-containing gas supply unit, the control unit, in a
case where the shutdown of operation of the fuel cell stored in the
storage unit is emergency shutdown, controlling the fuel gas supply unit
at a time of restarting the fuel cell after the shutdown of operation of
the fuel cell so as to reduce an amount of fuel gas supplied to the fuel
cell to be less than an amount of fuel gas supplied to the fuel cell at a
time of restarting the fuel cell after normal shutdown of operation of
the fuel cell.

12. The method of operating a fuel cell system according to claim 11,
wherein when the temperature in a vicinity of the fuel cell during
restarting of the fuel cell reaches a predetermined temperature, the
amount of fuel gas supplied to the fuel cell decreases to be less than an
amount of fuel gas supplied to the fuel cell at a temperature lower than
the predetermined temperature.

13. The method of operating a fuel cell system according to claim 12,
wherein the predetermined temperature is a temperature to change water
into steam.

14. The method of operating a fuel cell system according to claim 12,
wherein the fuel cell includes a fuel battery cell having a solid
electrolyte, and an oxygen electrode layer and a fuel electrode layer
containing a metal which are disposed to interpose the solid electrolyte
therebetween, and the predetermined temperature is a reduction
temperature of an oxide of the metal constituting the fuel electrode
layer.

15. The method of operating a fuel cell system according to claim 12,
wherein the fuel cell system further comprises an ignitor that is
disposed on the first end portion side of the fuel cell and combusts the
fuel gas remaining unused for generation of electric power, and the
ignitor is operated when the temperature during restarting of the fuel
cell reaches the predetermined temperature.

16. The method of operating a fuel cell system according to claim 12,
wherein the fuel cell has a configuration in which a plurality of
strip-like fuel battery cells are arranged in line in an arrangement
direction perpendicular to a longitudinal direction of the fuel battery
cells and the plurality of fuel battery cells are electrically connected
to each other.

17. The method of operating a fuel cell system according to claim 16,
wherein the fuel cell system further comprises a temperature detecting
unit that measures a temperature in a vicinity of the fuel cell, and the
temperature detecting unit is disposed at a center in the arrangement
direction of the fuel battery cells in the fuel cell and at a center in
the longitudinal direction of the fuel battery cells.

18. The method of operating a fuel cell system according to claim 16,
wherein the fuel cell system further comprises a temperature detecting
unit that measures a temperature in a vicinity of the fuel cell, and the
temperature detecting unit is disposed at an end in the arrangement
direction of the fuel battery cells in the fuel cell and at a second end
portion opposite to a first end portion of the fuel cell.

19. The method of operating a fuel cell system according to claim 12,
wherein the fuel cell is a solid oxide fuel cell.

Description:

FIELD OF INVENTION

[0001] The present invention relates to a fuel cell system and an
operating method thereof.

BACKGROUND

[0002] Conventionally, a fuel cell system is known which uses a
high-temperature operating fuel cell such as a solid oxide fuel cell
(SOFC) or a molten carbonate fuel cell (MCFC). A solid oxide fuel cell
(SOFC) operates in a high temperature range of about 600° C. to
1000° C. and a molten carbonate fuel cell (MCFC) operates in a
high temperature range of about 500° C. to 900° C.

[0003] Conventionally, when the supply of fuel gas is stopped due to
natural disasters such as an earthquake, a power failure, a lightning
strike, or a typhoon, a fuel cell emergently shuts down (for example, see
Patent Literature 1).

[0004] Conventionally, after the operation of a fuel cell is stopped,
maintenance or the like is performed and the operation of the fuel cell
is restarted (for example, see Patent Literature 2).

[0007] However, the shutdown of operation of the fuel cell includes normal
shutdown and emergency shutdown as described above. When a fuel cell
emergently shuts down and is then restarted in the same way as in the
normal shutdown, the fuel cell may be adversely influenced.

[0008] An object of the invention is to provide a fuel cell system and an
operating method thereof, capable of reducing an adverse influence which
acts on a fuel cell at the time of restarting the fuel cell after
emergency shutdown of operation of the fuel cell.

Solution to Problem

[0009] A fuel cell system according to the invention includes: a fuel cell
that generates electric power using fuel gas and oxygen-containing gas,
and combusts fuel gas remaining unused for generation of electric power,
on a first end portion side thereof; a fuel gas supply unit that supplies
the fuel gas to the fuel cell; an oxygen-containing gas supply unit that
supplies the oxygen-containing gas to the fuel cell; a storage unit that,
if any shutdown of operation of the fuel cell, stores whether the
shutdown of operation of the fuel cell is normal shutdown or emergency
shutdown; and a control unit that controls at least the fuel gas supply
unit and the oxygen-containing gas supply unit, the control unit, in a
case where the shutdown of operation of the fuel cell stored in the
storage unit is emergency shutdown, controlling the fuel gas supply unit
at a time of restarting the fuel cell after the shutdown of operation of
the fuel cell so as to reduce an amount of fuel gas supplied to the fuel
cell to be less than an amount of fuel gas supplied to the fuel cell at a
time of restarting the fuel cell after normal shutdown of operation of
the fuel cell.

[0010] A method of operating a fuel cell system according to the invention
includes controlling an amount of fuel gas supplied to a fuel cell that
generates electric power using fuel gas and oxygen-containing gas and
combusts fuel gas remaining unused for generation of electric power, at a
time of restarting the fuel cell after emergency shutdown of operation of
the fuel cell, to be less than an amount of fuel gas supplied to the fuel
cell at a time of restarting the fuel cell after normal shutdown of
operation of the fuel cell.

Advantageous Effects of Invention

[0011] In the fuel cell system according to the invention, in the case of
restarting a fuel cell after the emergency shutdown of operation of the
fuel cell, even though a fuel electrode layer of the fuel cell, a support
having the fuel electrode layer disposed therein, and the like are
oxidized, the fuel electrode layer, the support, and the like are slowly
reduced, and it is possible to suppress rapid generation of a stress in
the fuel cell without rapid volume contraction of the fuel electrode
layer, the support, and the like. It is thus possible to reduce adverse
influence which acts on the fuel cell at the time of restarting the fuel
cell after the emergency shutdown of operation of the fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a diagram illustrating a fuel cell system;

[0013] FIG. 2 is a diagram illustrating a solid oxide fuel battery cell,
where FIG. 2(a) is a transverse cross-sectional view and FIG. 2(b) is a
longitudinal cross-sectional view;

[0014] FIG. 3 is a diagram illustrating an example of a fuel cell, where
FIG. 3(a) is a side view schematically illustrating the fuel cell and
FIG. 3(b) is a partially-enlarged cross-sectional view illustrating parts
surrounded with dotted lines in the fuel cell in FIG. 3(a);

[0015] FIG. 4 is an exterior perspective view illustrating an example of a
fuel cell module;

[0016] FIG. 5 is an exploded perspective view illustrating an example of a
fuel cell device;

[0017] FIG. 6 is a flowchart illustrating a flow of a restarting operation
after emergency shutdown; and

[0018] FIG. 7(a) is a graph illustrating a relationship between a
temperature T of a lower end portion of the fuel cell and a startup time
t thereof and FIG. 7(b) is a graph illustrating a relationship between an
amount of fuel gas L supplied to the fuel cell and the startup time t
thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] FIG. 1 shows a fuel cell system according to an embodiment of the
invention, where reference numeral 1 represents a solid oxide fuel cell 1
(hereinafter, also referred to as a fuel cell 1). The fuel cell 1 has a
plurality of fuel battery cells electrically connected in series to each
other.

[0020] The fuel cell 1 generates electric power using fuel gas and
oxygen-containing gas. The fuel cell 1 is housed in a housing 3. The
housing 3 also houses a reformer 5 and is configured to supply the fuel
gas, which is reformed by the reformer 5, to the fuel cell 1. A catalyst
for reforming raw fuel gas such as city gas is received in the reformer
5. The fuel gas reformed by the reformer 5 contains steam. The fuel cell
1 will be described later.

[0021] The reformer 5 is configured to be supplied with raw fuel gas such
as city gas, which is reformed into fuel gas, via a security meter 7. A
security meter 7 is provided, for example, to each household. The usage
of raw fuel gas used in the corresponding household is displayed on the
security meter 7, and the security meter includes an on-off valve for
stopping the supply of raw fuel gas to the fuel cell system at the time
of emergency of natural disasters such as an earthquake, a power failure,
a lightning strike, or a typhoon. The security meter 7 is also a gas
meter. The security meter 7 is configured to supply the raw fuel gas to,
for example, a water heater and a gas stove, in addition to the fuel cell
system, and includes a gas-appliance raw fuel line 9c for each gas
appliance.

[0022] The reformer 5 and the fuel cell 1 are connected to each other with
a reformed fuel line 9a supplying the fuel gas. The security meter 7 and
the reformer 5 are connected to each other with a raw fuel line 9b
supplying the raw fuel gas. The reformed fuel line 9a and the raw fuel
line 9b constitute a fuel gas supply line. The reformed fuel line 9a, the
raw fuel line 9b, and the gas-appliance raw fuel line 9c are formed of
pipes. The security meter 7 having an on-off valve is disposed at an
upstream end of the fuel gas supply line 9, that is, at an upstream end
of the raw fuel line 9b.

[0023] A pump 11 supplying the raw fuel gas to the reformer 5 is disposed
downstream in the fuel gas supply line 9, specifically, downstream from
the on-off valve of the raw fuel line 9b. The pump 11 and the security
meter 7 are connected to a control unit 10. The control unit 10 controls
the amount of raw fuel gas supplied to the reformer 5 so as to control
the amount of fuel gas supplied to the fuel cell 1 depending on a load,
by controlling the pump 11.

[0024] The load means electrical appliances such as a refrigerator, a
washing machine, or a microwave oven and is supplied with electric power
generated by the fuel cell 1.

[0025] In FIG. 1, the gas-appliance raw fuel lines 9c are connected to the
water heater and the gas stove, respectively. The gas-appliance raw fuel
lines 9c are connected to the raw fuel line 9b of the fuel cell system.

[0026] A check valve (not shown) which is opened only at a pressure higher
than a predetermined pressure is disposed downstream from the pump 11 in
the fuel gas supply line 9, and stops the supply of raw fuel gas when the
pump 11 shuts down. The fuel gas supply unit includes the pump 11.

[0027] The reformer 5 is supplied with water by the use of a pump (not
shown) controlled by the control unit 10. The water is changed to steam
in the reformer 5 and reacts with the raw fuel gas such as city gas and
propane gas to enable steam reforming.

[0028] The fuel cell 1 is supplied with oxygen-containing gas such as air
via an oxygen-containing gas supply line 12. The supply of
oxygen-containing gas to the fuel cell 1 is performed by a blower 13
disposed in the oxygen-containing gas supply line 12. The blower 13 is
controlled by the control unit 10. The oxygen-containing gas supply unit
includes the blower 13.

[0029] An ignitor 16 such as a heater is disposed above the fuel cell 1
and ignites the fuel gas remaining unused for generation of electric
power. The ignition timing of the ignitor 16 is controlled by the control
unit 10.

[0030] In the fuel cell 1, a temperature sensor 17 such as a thermocouple
measuring the temperature in the vicinity of the fuel cell 1 is disposed
at the center in the longitudinal direction of the fuel battery cells and
the center in an arrangement direction of the plurality of fuel battery
cells, as described later, and a signal from the temperature sensor 17 is
transmitted to the control unit 10. The temperature detecting unit
includes the temperature sensor 17.

[0031] In a steady state (power generation mode), the fuel gas is supplied
from the reformer 5 to the fuel cell 1 and the oxygen-containing gas
(hereinafter, air may also be described as the oxygen-containing gas)
such as air is supplied to the housing 3, whereby the fuel cell 1
generates electric power. The fuel gas remaining unused for generation of
electric power is combusted on the reformer 5 side in the fuel cell 1 by
reacting with the oxygen-containing gas, and a combustion area 15 is
formed between the fuel cell 1 and the reformer 5. The reformer 5 is
heated by the combustion gas and the raw fuel gas in the reformer 5 is
reformed into fuel gas containing hydrogen as a major component.

[0032] The fuel cell 1 has a plurality of fuel battery cells electrically
connected in series to each other and a fuel battery cell is shown in
FIG. 2. FIG. 2(a) is a transverse cross-sectional view of a fuel battery
cell 20 and FIG. 2(b) is a longitudinal cross-sectional view of the fuel
battery cell 20. In both drawings, the fuel battery cell 20 is partly
enlarged.

[0033] The fuel battery cell 20 is a fuel battery cell 20 having a hollow
plate shape and includes a porous conductive support substrate
(hereinafter, also referred to as a conductive support) 21 having a flat
cross-section and an elliptic cylinder shape as a whole. A plurality of
fuel gas flow channels 22 are formed at an appropriate interval so as to
penetrate the conductive support 21 in the longitudinal direction y. The
fuel battery cell 20 has a structure in which various members are formed
on the conductive support 21.

[0034] The conductive support 21 includes a pair of flat surfaces n
parallel to each other and arc-like surfaces (side surfaces) m connecting
the pair of flat surfaces n to each other. Both flat surfaces n are
substantially parallel to each other, and a porous fuel electrode layer
23 is formed to cover one flat surface n (lower surface) and both
arc-like surfaces m. A dense solid electrolyte layer 24 is stacked
thereon to cover the fuel electrode layer 23. A porous oxygen electrode
layer 26 is stacked on the solid electrolyte layer 24 so as to face the
fuel electrode layer 23 with a reaction preventing layer 25 interposed
therebetween. An interconnector 28 is formed on the other flat surface n
(upper surface), on which the fuel electrode layer 23 and the solid
electrolyte layer 24 are not stacked, with a close adhesive layer 27
interposed therebetween.

[0035] The conductive support 21 and the fuel electrode layer 23 contain
metal, and Ni, Fe, and Co, and the like are known as examples of the
metal. The fuel electrode layer 23 contains metal oxide in addition to
metal, and stabilized zirconia or partially-stabilized zirconia is known
as examples of the metal oxide.

[0036] That is, the fuel electrode layer 23 and the solid electrolyte
layer 24 are formed to the other flat surface n (upper surface) via the
arc-like surfaces m on both sides of the conductive support 21, both ends
of the interconnector 28 are located at both ends of the solid
electrolyte layer 24, the conductive support 21 is surrounded with the
solid electrolyte layer 24 and the interconnector 28 so as not to leak
the fuel gas flowing therein to the outside.

[0037] As indicated by an arrow in FIG. 2(b), the fuel gas flows from a
lower end to an upper end of the fuel gas flow channel 22 of each fuel
battery cell 20, is discharged to the upper side from the fuel battery
cell 20, and is combusted above the fuel battery cell 20. Therefore, an
upper end portion which is the combustion side of the fuel battery cell
20 is higher in temperature than the lower end portion thereof.

[0038] FIG. 3 is a diagram illustrating an example of a fuel cell (also
referred to as a cell stack) in which the plurality of fuel battery cells
20 as described above are electrically connected in series with a power
collecting member 33 interposed therebetween. FIG. 3(a) is a side view
schematically illustrating the fuel cell 1, and FIG. 3(b) is a
partially-enlarged cross-sectional view of the fuel cell 1 in FIG. 3(a)
and shows the enlargement of the parts surrounded by dotted lines in FIG.
3(a). In FIG. 3(b), portions corresponding to the parts surrounded by the
dotted lines in FIG. 3(a) are guided by arrows so as to specify the
portions. In the fuel battery cells 20 shown in FIG. 3(b), some members
such as the reaction preventing layer 25 are not shown.

[0039] In the fuel cell 1, the fuel battery cells 20 are arranged in a
line with the power collecting member 33 interposed therebetween in the
direction (in the thickness direction of the fuel battery cells 20)
perpendicular to the longitudinal direction of the fuel battery cells 20
to form the fuel cell 1, and the lower end portion of each fuel battery
cell 20 is fixed to a gas tank 36 supplying the fuel gas to the fuel
battery cells 20 with an adhesive such as a glass sealant. Elastically
deformable conductive members 34 whose lower end portions are fixed to
the gas tank 36 are provided so as to pinch the plurality of fuel battery
cells 20 from both ends in the arrangement direction x of the fuel
battery cells 20. Here, the fuel cell 1 has a structure in which a
plurality of fuel battery cells 20 are fixed to the gas tank 36.

[0040] In the conductive member 34 shown in FIG. 3, a current drawing
portion 35 for drawing out current generated by the power generation of
the fuel cell 1 is disposed in a shape extending to the outside along the
arrangement direction x of the fuel battery cells 20. The temperature
sensor 17 is disposed at the center in the arrangement direction x of the
plurality of fuel battery cells 20 and at the center in the longitudinal
direction y of each fuel battery cell 20. The temperature sensor 17 is
disposed at a position not contacting but as close as possible to the
fuel battery cells 20.

[0041] FIG. 4 is an exterior perspective view illustrating an example of a
fuel cell module 38 in which the fuel cell 1 is housed in the housing 3,
where the fuel cell 1 shown in FIG. 3 is housed in the housing 3 having a
rectangular parallelepiped shape.

[0042] The reformer 5 is disposed above the fuel cell 1 so as to obtain
the fuel gas used in the fuel battery cells 20. The fuel gas generated in
the reformer 5 is supplied to the gas tank 36 via the fuel gas supply
line 9a and is supplied to the fuel gas flow channels 22 disposed in the
fuel battery cells 20 via the gas tank 36. In FIG. 4, the fuel cell 1 has
a structure in which the fuel battery cells 20 are arranged in two lines
and fixed to the gas tank 36.

[0043] FIG. 4 shows a state where a part (front and rear surfaces) of the
housing 3 is removed and the fuel cell 1 and the reformer 5 housed
therein are drawn out to the rear side. In the fuel cell module 38 shown
in FIG. 4, the fuel cell 1 can be housed in the housing 3 in a sliding
manner.

[0044] An oxygen-containing gas introducing member 37 disposed in the
housing 3 is disposed between cell groups of two lines installed in
parallel to the gas tank 36 in FIG. 4. Oxygen-containing gas is supplied
to the lower end portions of the fuel battery cells 20 by the use of the
oxygen-containing gas introducing member 37 so that the oxygen-containing
gas flows from a lower end to an upper end through sides of the fuel
battery cells 20 to correspond to the flow of the fuel gas. The
oxygen-containing gas introducing member 37 constitutes a part of the
oxygen-containing gas supply line 12.

[0045] By causing the fuel gas discharged from the fuel gas flow channels
22 of the fuel battery cells 20 to react with the oxygen-containing gas
so as to combust the fuel gas above the fuel battery cells 20, it is
possible to raise the temperature of the fuel battery cells 20 and thus
to promote the startup of the fuel cell 1. By causing the fuel gas
discharged from the gas flow channels 22 of the fuel battery cells 20 to
react with the oxygen-containing gas above the fuel battery cells 20 so
as to combust the fuel gas, the reformer 5 disposed above the fuel
battery cells 20 (the fuel cell 1) can be heated. Accordingly, it is
possible to efficiently perform the reforming reaction in the reformer 5.
The reformer 5 is rapidly heated by the combustion of the fuel gas at the
time of startup, but the temperature rise of the lower end portions of
the fuel battery cells 20 into which the fuel gas is introduced is slow.

[0046] That is, the fuel gas introduction side (second end portion) in the
longitudinal direction y of the fuel cell 1 is lower in temperature than
the fuel gas combustion side (first end portion). Particularly, the
temperature difference increases at the time of startup.

[0047] FIG. 5 is an exploded perspective view illustrating an example of a
fuel cell device in which the fuel cell module 38 shown in FIG. 4 and
auxiliary units for operating the fuel cell 1 are housed in an exterior
case. Some elements are not shown in FIG. 5.

[0048] The fuel cell device 43 shown in FIG. 5 includes an exterior case
having columns 44 and exterior plates 45, and the exterior case is
vertically partitioned by a partition plate 46. An upper side space of
the exterior plates 45 is defined as a module receiving chamber 47
receiving the fuel cell module 38 and a lower side space is defined as an
auxiliary unit receiving chamber 48 receiving auxiliary units used to
operate the fuel cell module 38. The auxiliary units received in the
auxiliary unit receiving chamber 48 are not shown. The pump 11 is
received in the auxiliary unit receiving chamber 48.

[0049] An air flow port 49 allowing air of the auxiliary unit receiving
chamber 48 to flow into the module receiving chamber 47 is formed in the
partition plate 46, and an exhaust port 50 exhausting air in the module
receiving chamber 47 is formed in a part of the exterior plate 45
constituting the module receiving chamber 47.

[0050] The fuel cell system according to this embodiment includes a
storage unit that, if any shutdown of operation of the fuel cell should
occur, stores whether the shutdown of operation of the fuel cell 1 is
normal shutdown or emergency shutdown, and a control unit 10 that, in a
case where the shutdown of operation of the fuel cell stored in the
storage unit is emergency shutdown, controls the fuel gas supply unit so
as to reduce an amount of fuel gas supplied to the fuel cell 1 at a time
of restarting the fuel cell after the shutdown of operation of the fuel
cell to be less than an amount of fuel gas supplied to the fuel cell at a
time of restarting the fuel cell after the normal shutdown of operation
of the fuel cell 1.

[0051] The first end portion on the combustion side of the fuel cell 1
means the upper end portion of the fuel cell 1 in FIG. 3(a) and the
second end portion on the opposite side of the first end portion means
the lower end portion of the fuel cell 1.

[0052] The storage unit that, if any shutdown of operation of the fuel
cell should occur, stores whether the shutdown of operation of the fuel
cell 1 is the normal shutdown or the emergency shutdown may be disposed
independently of the control unit 10. Alternatively, the control unit 10
may have the same function as the storage unit.

[0053] The normal shutdown for stopping the operation (power generation)
of the fuel cell 1 is performed, for example, by pressing a stop button.
For example, after the supply of the oxygen-containing gas is stopped,
the fuel gas is supplied for a predetermined time or the housing 3 is
filled with inert gas or the like instead of the oxygen-containing gas,
so as not to oxidize the fuel electrode layer 23 of the fuel cell 1. When
the fuel cell 1 is restarted after the normal shutdown, the restart is
performed, for example, by pressing a restart switch, and the fuel gas
supplied to the fuel cell 1 is combusted to rapidly heat the fuel cell 1
to a temperature at which electric power can be generated.

[0054] On the other hand, in the emergency shutdown, for example, when the
on-off valve of the security meter 7 is closed and the supply of raw fuel
gas to the reformer 5 is stopped due to natural disasters such as an
earthquake, a power failure, a lightning strike, or a typhoon,
malfunctions of the system, and the like, the control unit 10 stops the
supply of power to a load and stops the operation of the pump 11 disposed
in the fuel gas supply line 9 at the time point of transmitting a signal
representing that the on-off valve is closed to the control unit 10. The
control unit 10 controls a water supply unit and an oxygen-containing gas
supply unit to stop the supply of water to the reformer 5 and the supply
of oxygen-containing gas to the fuel cell 1. Similarly, when the supply
of oxygen-containing gas and the supply of water to the reformer are
stopped, the fuel cell system emergently shuts down similarly.

[0055] For example, in the normal operation in which the electric power
generated by the fuel cell 1 is supplied to an electric appliance, the
control unit 10 controls the pump 11, the blower 13, and the like so as
to supply the fuel gas at a flow rate of 2.3 L/min, the air at a flow
rate of 50 L/min, and the water at an S/C of 2.5. In the emergency
shutdown, the supplies of the fuel gas, the air, and the water are
immediately stopped.

[0056] However, in the high-temperature operating fuel cell 1, when the
shutdown of operation of the fuel cell 1 is the emergency shutdown, the
supplies of the fuel gas, the air, and the water are immediately stopped
and thus the supply of fuel gas to the fuel electrode layer 23 of the
fuel cell 1 is rapidly stopped. In this state, since the fuel cell 1 is
cooled, an inside of the housing 3 is under a negative pressure, and thus
external air may be introduced via a combustion gas discharge port or the
like and may be introduced into the fuel cell 1. Accordingly, the fuel
electrode layer 23 of the fuel cell 1, the support 21, etc. may be
oxidized. In this state, the temperature of the fuel cell 1 is lowered to
a room temperature and the operation of the fuel cell 1 is stopped.

[0057] At the time of restarting the operation of the fuel cell 1, the
fuel gas is supplied to the fuel electrode layer 23 of the fuel cell 1
and the fuel gas remaining unused for generation of the electric power is
combusted to rapidly heat the fuel cell 1, in the state where the
temperature of the fuel cell 1 is lowered to the room temperature.
Accordingly, the reduction of the oxidized fuel electrode layer 23 of the
fuel cell 1 rapidly proceeds and a large stress can be easily applied to
the fuel cell 1.

[0058] Therefore, in this embodiment, the control unit 10, in a case where
the shutdown of operation of the fuel cell 1 stored in the storage unit
is the emergency shutdown, controls the fuel gas supply unit at the time
of restarting the fuel cell after the shutdown of operation of the fuel
cell so as to reduce an amount of fuel gas supplied to the fuel cell 1 to
be less than that at the time of restarting the fuel cell after the
normal shutdown of operation of the fuel cell 1.

[0059] In this embodiment, in the entire restarting step after the
emergency shutdown of operation of the fuel cell 1, the amount of fuel
gas supplied may be set to be lower than that at the time of restarting
the fuel cell after the normal shutdown of operation of the fuel cell 1.
In this case, even when the fuel electrode layer 23 of the fuel cell 1,
the conductive support 21 having the fuel electrode layer 23 disposed
thereon, or the like has been oxidized, the fuel electrode layer 23, the
conductive support 23, or the like is slowly reduced and thus does not
undergo rapid volume contraction. Accordingly, it is possible to suppress
rapid application of a stress to the fuel cell 1 and to reduce the
adverse influence which acts on the fuel cell 1 at the time of restarting
fuel cell after the emergency shutdown of operation of the fuel cell 1.
As described below, at least some steps of the restarting step may be
performed in a fuel gas supply decreasing mode.

[0060] On the other hand, since the fuel gas is combusted at the time of
restarting the fuel cell 1, the temperature of the second end portion
(lower end portion) on the opposite side of the first end portion (upper
end portion) on the combustion side of the fuel cell 1 is lower than that
of the first end portion (upper end portion) on the combustion side of
the fuel cell 1. Particularly, when the temperature of the lower end
portion of the fuel cell 1 is less than the temperature to change water
in the fuel gas into steam and the fuel gas is supplied to the lower end
portion of the fuel cell 1 at the lower temperature, the steam in the
fuel gas is liquefied to water and the S/C of the fuel gas supplied to
the end portion (upper end portion) on the combustion side of the fuel
cell 1 is lower than the designed value, carbon may be deposited on the
fuel electrode layer 23 or the conductive support 21 having the fuel
electrode layer 23 disposed thereon.

[0061] In other words, when the temperature of the lower end portion of
the fuel cell 1 is equal to or higher than the temperature to change
water into steam, for example, a high temperature equal to or higher than
100° C., the steam in the fuel gas is supplied to the fuel
electrode layer 23 at the fuel cell end portion on the combustion side
and the designed S/C is achieved. However, when the temperature of the
lower end portion of the fuel cell 1 is lower than the temperature to
change water into steam, the steam in the fuel gas is liquefied, the S/C
in the fuel gas supplied to the upper end portion on the combustion side
of the fuel cell 1 is lowered, carbon is deposited on the fuel electrode
layer 23 or the conductive support 21 having the fuel electrode layer 23
disposed thereon, and thus the fuel cell 1 may be adversely influenced.
The temperature to change water into steam is in a range of 95° C.
to 105° C.

[0062] Therefore, in this embodiment, when the temperature during
restarting of the fuel cell 1 which is detected by the temperature sensor
17 reaches a predetermined temperature, the control unit 10 controls the
fuel gas supply unit so as to reduce the amount of fuel gas supplied to
the fuel cell 1 to be less than that at a temperature lower than the
predetermined temperature. For example, when the temperature sensor 17 is
equal to or higher than the temperature to change water into steam, the
fuel gas supply unit is controlled so as to reduce the amount of fuel gas
supplied.

[0063] That is, as shown in FIG. 6, when the shutdown of operation of the
fuel cell 1 is the emergency shutdown, the control unit 10 heats the fuel
cell 1 at a high rate in a rapid temperature-rising mode, for example,
until the lower end portion of the fuel cell 1 reaches the temperature to
change water in the fuel gas into steam. After the lower end portion of
the fuel cell 1 reaches the temperature to change water in the fuel gas
into steam, the control unit 10 sets the fuel gas supply decreasing mode
so as to reduce the amount of fuel gas supplied to the fuel cell 1 to be
less than before, to lower the temperature-rising rate of the fuel cell 1
by the combustion of the fuel gas remaining unused for generation of
electric power, and to slowly heat the fuel cell 1.

[0064] In other words, until the lower end portion of the fuel cell 1
reaches the temperature to change water in the fuel gas into steam after
the startup, the supply of fuel gas is increased to increase the amount
of fuel gas combusted above the fuel battery cells 20 and to rapidly
raise the temperature of the fuel cell 1. After the lower end portion of
the fuel cell 1 reaches the temperature to change water in the fuel gas
into steam, the supply of fuel gas is decreased to reduce the amount of
fuel gas combusted above the fuel battery cells 20 and to slowly raise
the temperature of the fuel cell 1. The amount of fuel gas supplied is
controlled on the basis of the temperature of the lower end portion of
the fuel cell 1, but substantially the same effects can be achieved even
on the basis of the temperature in the vicinity of the fuel cell 1.

[0065] When the fuel cell 1 has a configuration in which a plurality of
strip-like fuel battery cells 20 are arranged in line in a direction
perpendicular to the longitudinal direction of the fuel battery cells 20
and each fuel battery cell 20 has the fuel gas flow channels 22 for
supplying fuel gas in the longitudinal direction of the fuel battery cell
20, a temperature difference can be generated in the longitudinal
direction of the fuel battery cells 20 and thus the invention can be
suitably applied thereto.

[0066] FIG. 7(a) shows the relationship between the temperature T of the
lower end portion of the fuel cell 1 and the startup time t and FIG. 7(b)
shows the relationship between the amount of fuel gas L supplied to the
fuel cell 1 and the startup time t.

[0067] The restart in this embodiment will be described in detail on the
basis of FIGS. 6 and 7. First, for example, by pressing a restart switch,
a restart instruction is transmitted to the control unit 10, and the
control unit 10 determines whether the shutdown of operation of the fuel
cell 1 just before the restart is the normal shutdown or the emergency
shutdown, using the storage unit. In the case of the normal shutdown, a
normal startup operation is performed. That is, the control unit 10
controls the pump 11, the blower 13, and the like so as to supply the
fuel gas at a flow rate of 2.5 L/min, the air at a flow rate of 40 L/min,
and the water at an S/C of 2.5. In the case of the normal shutdown, since
the fuel cell shuts down so as not to oxidize the fuel electrode layer of
the fuel cell, the rapid restart in the rapid temperature-rising mode
does not damage the fuel cell 1. The normal startup is indicated by a
broken line in FIG. 7.

[0068] On the other hand, in the case of the emergency shutdown, until the
lower end portion of the fuel cell 1 reaches the temperature to change
water in the fuel gas into steam, for example, 100° C., the
control unit 10 controls the pump 11, the blower 13, and the like so as
to supply the fuel gas at a flow rate of 2.5 L/min, the air at a flow
rate of 40 L/min, and the water at an S/C of 2.5 and operates the fuel
cell in the rapid temperature-rising mode. After the lower end portion of
the fuel cell 1 reaches the temperature to change water in the fuel gas
into steam, the fuel cell is operated in the fuel gas supply decreasing
mode. In the fuel gas supply decreasing mode, the fuel gas is preferably
equal to or less than 80% of the amount of fuel gas in the rapid
temperature-rising mode and more preferably equal to or less than 60%.
For example, the control unit 10 controls the pump 11, the blower, and
the like so as to supply the fuel gas at a flow rate of 1.2 L/min, the
air at a flow rate of 40 L/min, and the water at an S/C of 2.5.

[0069] The temperature T of the second end portion on the opposite side of
the first end portion on the combustion side of the fuel cell 1, for
example, as in FIG. 3(a), the lower end portion of the fuel cell 1, is
obtained from the signal of the temperature sensor 17. That is, the
temperature sensor 17 is disposed at the center in the arrangement
direction x of the plurality of fuel battery cells 20 and at the center
in the longitudinal direction y of the fuel battery cells 10. The
temperature sensor 17 is disposed in a portion having the highest
temperature in the fuel cell 1 and constantly monitors the temperature of
the fuel cell 1. The temperature of the lower end portion of the fuel
cell 1 is calculated on the basis of the signal from the temperature
sensor 17 by acquiring the correlation between the temperature at the
center in the longitudinal direction y of the fuel battery cells 10 and
the temperature of the lower end portion of the fuel cell 1 at the center
in the arrangement direction x of the plurality of fuel battery cells 20
in advance.

[0070] The temperature of the fuel cell 1 is low in the lower end portions
of the fuel battery cells 10 because they are separated apart from the
combustion region 15, and is low in both end portions in the arrangement
direction x of the fuel battery cells 10 because heat can be easily
dissipated. Accordingly, as indicated by a circle mark in FIG. 3(a), it
is preferable that the temperature sensor 17 be disposed at an end in the
arrangement direction x of the fuel battery cells 10 and in the lower end
portion of the fuel cell 1, the temperature of a portion having the
lowest temperature be calculated, and the fuel gas supply decreasing mode
be entered on the basis of this temperature.

[0071] In this embodiment as mentioned above, when the temperature of the
fuel cell 1 is equal to or higher than the temperature to change water in
the fuel gas into steam, the temperature of the fuel cell 1 is slowly
raised in the fuel gas supply decreasing mode. Accordingly, even when the
fuel electrode layer 23 or the conductive support 21 of the fuel cell 1
is oxidized, the fuel electrode layer 23 or the conductive support 21 is
slowly reduced and the rapid volume contraction thereof is suppressed. As
a result, it is possible to suppress the rapid application of a stress to
the fuel cell 1 and to reduce the adverse influence which acts on the
fuel cell 1 at the time of restarting the fuel cell after the emergency
shutdown of operation of the fuel cell 1.

[0072] The temperature of the entire fuel cell 1 can be rapidly raised to
not lower than the temperature to change water in the fuel gas into
steam, the S/C in the fuel gas supplied to the upper end portion on the
combustion side of the fuel cell 1 can be kept high, and the deposition
of carbon in the fuel electrode layer 23 or the conductive support 21
having the fuel electrode layer 23 disposed thereon can be prevented or
reduced.

[0073] The temperature to change water in the fuel gas into steam is about
100° C., but, from the viewpoint of shortening of the startup
time, control may be performed to enter the fuel gas supply decreasing
mode at a temperature equal to or higher than a reduction temperature of
an oxide of a metal constituting the fuel electrode layer 23 of the fuel
battery cell 10, which is higher than 100° C. For example, when
the metal constituting the fuel electrode layer 23 is Ni, a reduction
temperature of NiO to Ni is equal to or higher than about 250° C.
Accordingly, the rapid temperature-rising mode is set up to 250°
C. and the fuel gas supply decreasing mode is set at a temperature equal
to or higher than 250° C. This state is additionally shown in FIG.
7(a).

[0074] As a result, it is possible to maintain the S/C in the fuel gas
supplied to the upper end portion on the combustion side of the fuel cell
1 as designed and to shorten the restart time because the temperature of
the fuel cell 1 is rapidly raised to the reduction temperature of the
oxide of the metal constituting the fuel electrode layer 23, for example,
NiO. Since the temperature of the fuel cell is slowly raised at a
temperature equal to or higher than the reduction temperature of the
oxide of the metal constituting the fuel electrode layer 23, it is
possible to suppress the rapid volume contraction of the fuel electrode
layer 23 and thus to suppress rapid application of a stress to the fuel
cell 1.

[0075] In the fuel gas supply decreasing mode, it is preferable that the
ignitor 16 be operated. Since the supply of fuel gas decreases in the
fuel gas supply decreasing mode, the combustion flame of the fuel
remaining unused for generation of electric power tends to be
extinguished, but it is possible to prevent fire from being extinguished
by constantly operating the ignitor 16 in the fuel gas supply decreasing
mode.

[0076] The invention is not limited to the above-mentioned embodiment, but
various modifications and changes are possible without departing from the
scope of the invention.

[0077] In the above-mentioned embodiment, the fuel cell employing hollow
plate-like solid oxide fuel battery cells is exemplified, but a fuel cell
employing cylindrical solid oxide fuel battery cells may be used. The
invention can be also applied to such a type of plate-like fuel cell in
which plural stacked members of a plate-like fuel battery cell in which a
fuel electrode layer, a solid electrolyte layer, and an oxygen electrode
layer are sequentially stacked, a fuel-side interconnector connected to
the fuel electrode layer, and an oxygen-side interconnector connected to
the oxygen electrode layer are stacked with a partition plate interposed
therebetween, fuel gas and oxygen-containing gas are supplied to the
central portions of the fuel electrode layer and the oxygen electrode
layer and flow to the outer circumference of the fuel electrode layer and
the oxygen electrode layer, and surplus fuel gas and oxygen-containing
gas are discharged and combusted from the outer circumference of the fuel
battery cells.